Lead electrode for use in an MRI-safe implantable medical device

ABSTRACT

A medical lead is configured to be implanted into a patient&#39;s body and comprises a lead body, and an electrode coupled to the lead body. The electrode comprises a first section configured to contact the patient&#39;s body, and a second section electrically coupled to the first section and configured to be capacitively coupled to the patient&#39;s body.

FIELD OF THE INVENTION

The present invention generally relates to implantable medical devices,and more particularly to an implantable MRI-safe lead of the type whichincludes a stimulation electrode for use in conjunction with animplantable medical device such as a neurostimulation system that whenused in an MRI environment conveys energy induced at MRI frequencies toa patient's body in a safe manner.

BACKGROUND OF THE INVENTION

Implantable medical devices are commonly used today to treat patientssuffering from various ailments. Such implantable devices may beutilized to treat conditions such as pain, incontinence, sleepdisorders, and movement disorders such as Parkinson's disease andepilepsy. Such therapies also appear promising in the treatment of avariety of psychological, emotional, and other physiological conditions.

One known type of implantable medical device, a neurostimulator,delivers mild electrical impulses to neural tissue using an electricallead. For example, to treat pain, electrical impulses may be directed tospecific sites. Such neurostimulation may result in effective painrelief and a reduction in the use of pain medications and/or repeatsurgeries.

Typically, such devices are totally implantable and may be controlled bya physician or a patient through the use of an external programmer.Current systems generally include a non-rechargeable primary cellneurostimulator, a lead extension, and a stimulation lead, and the twomain classes of systems may be referred to as: (1) Spinal CordStimulation (SCS) and (2) Deep Brain Stimulation (DBS).

An SCS stimulator may be implanted in the abdomen, upper buttock, orpectoral region of a patient and may include at least one extensionrunning from the neurostimulator to the lead or leads which are placedsomewhere along the spinal cord. Each of the leads (to be discussed indetail hereinbelow) currently contains from one to eight electrodes.Each extension (likewise to be discussed in detail below) is pluggedinto or connected to the neurostimulator at a proximal end thereof andis coupled to and interfaces with the lead or leads at a distal end ofthe extension.

The implanted neurostimulation system is configured to send mildelectrical pulses to the spinal cord. These electrical pulses aredelivered through the lead or leads to regions near the spinal cord or anerve selected for stimulation. Each lead includes a small insulatedwire coupled to an electrode at the distal end thereof through which theelectrical stimulation is delivered. Typically, the lead also comprisesa corresponding number of internal wires to provide separate electricalconnection to each electrode such that each electrode may be selectivelyused to provide stimulation. Connection of the lead to an extension maybe accomplished by means of a connector block including, for example, aseries or combination of set screws, ball seals, etc. The leads areinserted into metal set screw bocks, and the metal set screws aremanipulated to press the contacts against the blocks to clamp them inplace and provide electrical connection between the lead wires and theblocks. Such an arrangement is shown in U.S. Pat. No. 5,458,629 issuedOct. 17, 1995 and entitled “Implantable Lead Ring Electrode and Methodof Making”.

A DBS system comprises similar components (i.e. a neurostimulator, atleast one extension, and at least one stimulation lead) and may beutilized to provide a variety of different types of electricalstimulation to reduce the occurrence or effects of Parkinson's disease,epileptic seizures, or other undesirable neurological events. In thiscase, the neurostimulator may be implanted into the pectoral region ofthe patient. The extension or extensions may extend up through thepatient's neck, and the leads/electrodes are implanted in the brain. Theleads may interface with the extension just above the ear on both sidesof the patient. The distal end of the lead may contain from four toeight electrodes and, as was the case previously, the proximal end ofthe lead may be connected to the distal end of the extension and may beheld in place by set screws. The proximal portion of the extension plugsinto the connector block of the neurostimulator.

Magnetic resonance imaging (MRI) is a relatively new and efficienttechnique that may be used in the diagnosis of many neurologicaldisorders. It is an anatomical imaging tool which utilizes non-ionizingradiation (i.e. no x-rays or gamma rays) and provides a non-invasivemethod for the examination of internal structure and function. Forexample, MRI permits the study of the overall function of the heart inthree dimensions significantly better than any other imaging method.Furthermore, imaging with tagging permits the non-invasive study ofregional ventricular function.

MRI scanning is widely used in the diagnosis of injuries to the head. Infact, the MRI is now considered by many to be the preferred standard ofcare, and failure to prescribe MRI scanning can be consideredquestionable. Approximately sixteen million MRIs were performed in 1996,followed by approximately twenty million in the year 2000. It isprojected that forty million MRIs will be performed in 2004.

In an MRI scanner, a magnet creates a strong magnetic field which alignsthe protons of hydrogen atoms in the body and then exposes them to radiofrequency (RF) energy from a transmitter portion of the scanner. Thisspins the various protons, and they produce a faint signal that isdetected by a receiver portion of the scanner. A computer renders thesesignals into an image. During this process, three electromagnetic fieldsare produced; i.e. (1) a static magnetic field, (2) a gradient magneticfield, and (3) a radio frequency (RF) magnetic field. The main or staticmagnetic field may typically vary between 0.2 and 3.0 Tesla. A nominalvalue of 1.5 Tesla is approximately equal to 15,000 Gauss which is30,000 times greater than the Earth's magnetic field of approximately0.5 Gauss. The time varying or gradient magnetic field may have amaximum strength of approximately 40 milli-Tesla/meters at a frequencyof 0-5 KHz. The RF may, for example, produce thousands of watts atfrequencies of between 8-215 MHz. For example, up to 20,000 watts may beproduced at 64 MHz and a static magnetic field of 1.5 Tesla; that is, 20times more power than a typical toaster. Thus, questions have arisenregarding the potential risk associated with undesirable interactionbetween the MRI environment and the above-described neurostimulationsystems; e.g. forces and torque on the implantable device within the MRIscanner caused by the static magnetic field, RF-induced heating, inducedcurrents due to gradient magnetic fields, device damage, and imagedistortion. Of these interactions, the problems associated with inducedRF currents in the leads are most deserving of attention since it hasbeen found that the temperature in the leads can rise by as much as 25°Centigrade or higher in an MRI environment.

A similar problem occurs when a patient undergoes diathermy treatmentemploying RF energy to create eddy currents in the patient's tissue soas to heat the tissue and promote healing. In this environment, currentmay also be produced in the implanted lead causing undesirable heatingof the electrodes as described above.

Accordingly, it would be desirable to provide an implantable medicaldevice that may be safely operated in an MRI environment. It would befurther desirable to provide an implantable medical device such as a SCSor DBS neurostimulation system that may be operated in an MRIenvironment without the generation of significant undesirable heat inthe leads due to induced RF currents. It would be further desirable toprovide an MRI-safe, implantable lead that may be used in conjunctionwith known implantable medical devices wherein the stimulationelectrodes shunt the energy induced in the electrodes during an MRI scansafely to a patient's body, thereby reducing the generation of unwantedheat at the leads stimulation electrodes. Furthermore, other desirablefeatures and characteristics of the present invention will becomeapparent from the subsequent detailed description of the invention andthe appended claims, taken in conjunction with the accompanying drawingsand this background of the invention.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the invention there is provided a medical leadconfigured to be implanted into a patient's body, the lead comprising alead body, and an electrode coupled to the lead body. The electrodecomprises a first section configured to contact the patient's body, anda second section electrically coupled to the first section andconfigured to be capacitively coupled to the patient's body.

According to a further aspect of the invention there is provided amedical electrode assembly for use on a lead configured to be implantedin a patient's body. The electrode comprises a first section configuredto contact the patient's body, and a second section electrically coupledto the first section and configured to be capacitively coupled to thepatient's body.

According to a still further aspect of the present invention there isprovided a pulse stimulation system for implantation into a patient'sbody. The system comprises a pulse generator, a conductive filer havinga proximal end electrically coupled to the pulse generator and having adistal end, a lead body for housing the conductive filer, and astimulation electrode configured on the lead body and electricallycoupled to the distal end of the filer. The stimulation electrodecomprises a first section configured to contact the patient's body fordelivering stimulation thereto, and a second section electricallycoupled to the first section and configured to be capacitively coupledto the patient's body.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and:

FIG. 1 illustrates a typical spinal cord stimulation system implanted ina patient;

FIG. 2 illustrates a typical deep brain stimulation system implanted ina patient;

FIG. 3 is an isometric view of the distal end of the lead shown in FIG.2;

FIG. 4 is an isometric view of the distal end of the extension shown inFIG. 2;

FIG. 5 is an isometric view of an example of a connector screw blocksuitable for connecting the lead of FIG. 3 to the extension shown inFIG. 4;

FIG. 6 is a top view of the lead shown in FIG. 2;

FIGS. 7 and 8 are cross-sectional views taken along lines 7-7 and 8-8,respectively, in FIG. 6;

FIG. 9 is a top view of an alternate lead configuration;

FIGS. 10 and 11 are longitudinal and radial cross-sectional views of ahelically wound lead of the type shown in FIG. 6;

FIGS. 12 and 13 are longitudinal and radial cross-sectional views,respectively, of a cabled lead;

FIG. 14 is an exploded view of a neurostimulation system;

FIG. 15 is a cross-sectional view of the extension shown in FIG. 14taken along line 15-15;

FIGS. 16, 17, and 18 are front, end, and isometric views, respectively,of an electrode in accordance with the present invention;

FIG. 19 is a side view of the distal end of a stimulation leadincorporating the electrode shown in FIGS. 16, 17, and 18;

FIG. 20 is a side view of a further embodiment of an electrode inaccordance with the present invention;

FIG. 21 is a side view of a stimulation lead utilizing a floatingelectrode in accordance with the present invention;

FIGS. 22 is a cross-sectional view illustrating a rolled capacitor;

FIG. 23 is a side view of the distal end of a stimulation lead inaccordance with an embodiment of the present invention;

FIG. 24 is a cross-sectional view of a further embodiment of a rolledcapacitor;

FIG. 25 is a cross-sectional view of a discoital capacitor for use inconjunction with the present invention; and

FIG. 26 is an isometric view of yet another embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description of the invention is merely exemplaryin nature and is not intended to limit the invention or the applicationand uses of the invention. Furthermore, there is no intention to bebound by any theory presented in the preceding background of theinvention or the following detailed description of the invention.

FIG. 1 illustrates a typical SCS system implanted in a patient. As canbe seen, the system comprises a pulse generator such as an SCSneurostimulator 20, a lead extension 22 having a proximal end coupled toneurostimulator 20 as will be more fully described below, and a lead 24having proximal end coupled to the distal end of extension 22 and havinga distal end coupled to one or more electrodes 26. Neurostimulator 20 istypically placed in the abdomen of a patient 28, and lead 24 is placedsomewhere along spinal cord 30. As stated previously, neurostimulator 20may have one or two leads each having four to eight electrodes. Such asystem may also include a physician programmer and a patient programmer(not shown). Neurostimulator 20 may be considered to be an implantablepulse generator of the type available from Medtronic, Inc. and capableof generating multiple pulses occurring either simultaneously or onepulse shifting in time with respect to the other, and havingindependently varying amplitudes and pulse widths. Neurostimulator 20contains a power source and the electronics for sending precise,electrical pulses to the spinal cord to provide the desired treatmenttherapy. While neurostimulator 20 typically provides electricalstimulation by way of pulses, other forms of stimulation may be used ascontinuous electrical stimulation.

Lead 24 is a small medical wire having special insulation thereon andincludes one or more insulated electrical conductors each coupled attheir proximal end to a connector and to contacts/electrodes 26 at itsdistal end. Some leads are designed to be inserted into a patientpercutaneously (e.g. the Model 3487A Pisces—Quad® lead available fromMedtronic, Inc.), and some are designed to be surgically implanted (e.g.Model 3998 Specify® lead, also available form Medtronic, Inc.). Lead 24may contain a paddle at its distant end for housing electrodes 26; e.g.a Medtronic paddle having model number 3587A. Alternatively, electrodes26 may comprise one or more ring contacts at the distal end of lead 24as will be more fully described below.

While lead 24 is shown as being implanted in position to stimulate aspecific site in spinal cord 30, it could also be positioned along theperipheral nerve or adjacent neural tissue ganglia or may be positionedto stimulate muscle tissue. Furthermore, electrodes 26 may be epidural,intrathecal or placed into spinal cord 30 itself. Effective spinal cordstimulation may be achieved by any of these lead placements. While thelead connector at proximal end of lead 24 may be coupled directly toneurostimulator 20, the lead connector is typically coupled to leadextension 22 as is shown in FIG. 1. An example of a lead extension isModel 7495 available from Medtronic, Inc.

A physician's programmer (not shown) utilizes telemetry to communicatewith the implanted neurostimulator 20 to enable the physician to programand manage a patient's therapy and troubleshoot the system. A typicalphysician's programmer is available from Medtronic, Inc. and bears ModelNo. 7432. Similarly, a patient's programmer (also not shown) also usestelemetry to communicate with neurostimulator 20 so as to enable thepatient to manage some aspects of their own therapy as defined by thephysician. An example of a patient programmer is Model 7434® 3 EZPatient Programmer available from Medtronic, Inc.

Implantation of a neurostimulator typically begins with the implantationof at least one stimulation lead usually while the patient is under alocal anesthetic. While there are many spinal cord lead designs utilizedwith a number of different implantation techniques, the largestdistinction between leads revolves around how they are implanted. Forexample, surgical leads have been shown to be highly effective, butrequire a laminectomy for implantation. Percutaneous leads can beintroduced through a needle, a much easier procedure. To simplify thefollowing explanation, discussion will focus on percutaneous leaddesigns, although it will be understood by those skilled in the art thatthe inventive aspects are equally applicable to surgical leads. Afterthe lead is implanted and positioned, the lead's distal end is typicallyanchored to minimize movement of the lead after implantation. The lead'sproximal end is typically configured to connect to a lead extension 22.The proximal end of the lead extension is then connected to theneurostimulator 20.

FIG. 2 illustrates a DBS system implanted in a patient 40 and comprisessubstantially the same components as does an SCS; that is, at least oneneurostimulator, at least one extension, and at least one stimulationlead containing one or more electrodes. As can be seen, eachneurostimulator 42 is implanted in the pectoral region of the patient.Extensions 44 are deployed up through the patient's neck, and leads 46are implanted in the patient's brain is as shown at 48. As can be seen,each of the leads 46 is connected to its respective extension 44 justabove the ear on both sides of patient 40.

FIG. 3 is an isometric view of the distal end of lead 46. In this case,four ring electrodes 48 are positioned on the distal end of lead 46 andcoupled to internal conductors of filers (not shown) contained withinlead 46. Again, while four ring electrodes are shown in FIG. 3, it is tobe understood that the number of electrodes can vary to suit aparticular application. FIG. 4 is an isometric view of the distal end ofextension 44, which includes a connector portion 45 having four internalcontacts 47. The proximal end of the DBS lead is shown in FIG. 3, plugsinto the distal connector 45 of extension 44, and is held in place bymeans of, for example, a plurality (e.g. 4) of set screws 50. Forexample, referring to FIG. 5, lead 46 terminates in a series of proximalelectrical ring contacts 48 (only one of which is shown in FIG. 5). Lead46 may be inserted through an axially aligned series of openings 52(again only one shown) in screw block 54. With a lead 46 so inserted, aseries of set screws (only one shown) are screwed into block 54 to drivecontacts 48 against blocks 54 and secure and electrically couple thelead 46. It should be appreciated, however, that other suitable methodsfor securing lead 46 to extension 44 may be employed. The proximalportion of extension 44 is secured to neurostimulator 42 as is shown inFIGS. 1 and 2.

FIG. 6 is a top view of lead 46 shown in FIG. 2. FIGS. 7 and 8 arecross-sectional views taken along lines 7-7 and 8-8, respectively, inFIG. 6. Distal end 60 of lead 46 includes at least one electrode 62(four are shown). As stated previously, up to eight electrodes may beutilized. Each of electrodes 62 is preferably constructed as is shown inFIG. 8. That is, electrode 62 may comprise a conductive ring 71 on theouter surface of the elongate tubing making up distal shaft 60. Eachelectrode 62 is electrically coupled to a longitudinal wire 66 (shown inFIGS. 7 and 8) each of which extends to a contact 64 at the proximal endof lead 46. Longitudinal wires 66 may be of a variety of configurations;e.g. discreet wires, printed circuit conductors, etc. From thearrangement shown in FIG. 6, it should be clear that four conductors orfilers run through the body of lead 46 to electrically connect theproximal electrodes 64 to the distal electrodes 62. As will be furtherdiscussed below, the longitudinal conductors 66 may be spirallyconfigured along the axis of lead 46 until they reach the connectorcontacts.

The shaft of lead 46 preferably has a lumen 68 extending therethroughfor receiving a stylet that adds a measure of rigidity duringinstallation of the lead. The shaft preferably comprises a comparativelystiffer inner tubing member 70 (e.g. a polyamine, polyamide, highdensity polyethylene, polypropylene, polycarbonate or the like).Polyamide polymers are preferred. The shaft preferably includes acomparatively softer outer tubing member 72; e.g. silicon or othersuitable elastomeric polymer. Conductive rings 71 are preferably of abiocompatible metal such as one selected from the noble group of metals,preferably palladium, platinum or gold and their alloys.

FIG. 9 illustrates an alternative lead 74 wherein distal end 76 isbroader (e.g. paddle-shaped) to support a plurality of distal electrodes78. A lead of this type is shown in FIG. 1. As was the case with thelead shown in FIGS. 6,7, and 8, distal electrodes 78 are coupled tocontacts 64 each respectively by means of an internal conductor orfiler. A more detailed description of the leads shown in FIGS. 6 and 9may be found in U.S. Pat. No. 6,529,774 issued Mar. 4, 2003 and entitled“Extradural Leads, Neurostimulator Assemblies, and Processes of UsingThem for Somatosensory and Brain Stimulation”.

Leads of the type described above may be of the wound helix filer typeor of the cabled filer type. FIGS. 10 and 11 are longitudinal and radialcross-sectional views, respectively, of a helically wound lead of thetype shown in FIG. 6. The lead comprises an outer lead body 80; aplurality of helically wound, co-radial lead filers 82; and a styletlumen 84. As stated previously, a stylet is a stiff, formable insertplaced in the lead during implant so as to enable the physician to steerthe lead to an appropriate location. FIG. 10 illustrates four separate,co-radially wound filers 86, 88, 90, and 92 which are electricallyinsulated from each other and electrically couple a single electrode 62(FIG. 6) to a single contact 64 (FIG. 6).

As can be seen, lead filers 82 have a specific pitch and form a helix ofa specific diameter. The helix diameter is relevant in determining theinductance of the lead. These filers themselves also have a specificdiameter and are made of a specific material. The filer diameter,material, pitch and helix diameter are relevant in determining theimpedance of the lead. In the case of a helically wound lead, theinductance contributes to a frequency dependent impedance. FIGS. 12 and13 are longitudinal and radially cross-sectional views, respectively, ofa cabled lead. The lead comprises outer lead body 94, stylet lumen 96,and a plurality (e.g. four to eight) of straight lead filers 98. Itshould be understood that each straight filer 98 may, if desired, be ofa cable construction comprised of a plurality of insulated straightfilers; e.g. a center filer surrounded by an additional six filers.

FIG. 14 is an exploded view of a neurostimulation system that includesan extension 100 configured to be coupled between a neurostimulator 102and lead 104. The proximal portion of extension 100 comprises aconnector 106 configured to be received or plugged into connector block109 of neurostimulator 102. The distal end of extension 100 likewisecomprises a connector 110 including internal contacts 111 and isconfigured to receive the proximal end of lead 104 having contacts 112thereon. The distal end of lead 104 includes distal electrodes 114.

FIG. 15 is a cross-sectional view of extension 100. Lead extension 100has a typical diameter of 0.1 inch, which is significantly larger thanthat of lead 104 so as to make extension 100 more durable than lead 104.Extension 100 differs from lead 104 also in that each filer 106 in thelead body is helically wound or coiled in its own lumen 108 and notco-radially wound with the rest of the filers as was the case in lead104.

The diameter of typical percutaneous leads is approximately 0.05 inch.This diameter is based upon the diameter of the needle utilized in thesurgical procedure to deploy the lead and upon other clinical anatomicalrequirements. The length of such percutaneous SCS leads is based uponother clinical anatomical requirements and is typically 28 centimeters;however, other lengths are utilized to meet particular needs of specificpatients and to accommodate special implant locations.

Lead length is an important factor in determining the suitability ofusing the lead in an MRI environment. For example, the greater length ofthe lead, the larger the effective loop area that is impacted by theelectromagnetic field (e.g. the longer the lead, the larger theantenna). Furthermore, depending on the lead length, there can bestanding wave effects that create areas of high current along the leadbody. This can be problematic if the areas of high current are near thedistal electrodes.

Compared to the helically wound lead, the cable lead has smaller DCresistance because the length of the straight filer is less than that ofa coiled filer and the impedance at frequency is reduced because theinductance has been significantly reduced. It has been determined thatthe newer cabled filer designs tend to be more problematic in an MRIenvironment than do the wound helix filer designs. It should be notedthat straight filers for cable leads sometimes comprise braided strandedwire that includes a number of smaller strands woven to make up eachfiler. This being the case, the number of strands could be varied toalter the impedance.

As stated previously, the electromagnetic fields within an MRIenvironment produce RF currents in the leads that can result inundesirable temperature increases at the leads stimulation electrodes.It has been discovered that this temperature increase can be reduced byproviding stimulation electrodes having effective surface areas thatincrease in an MRI environment.

FIGS. 16, 17, and 18 are front, end, and isometric views of astimulation electrode 110 for use in an MRI-safe stimulation lead. Thestimulation electrode comprises a first ring portion 112 having apredetermined outer diameter substantially equal to the outer diameterof lead body 104 (FIG. 19). A second ring portion 116 extends fromportion 112 and has an outer diameter smaller than that of the firstring portion. Opening 118 extends through both ring portions to permitconductive filers (not shown) to pass there through.

FIG. 19 is a side view of the distal end of a stimulation lead 104 thathas positioned thereon four electrodes 110 of the type previouslydescribed in connection with FIGS. 16, 17, and 18. As can be seen, lead104 includes a lead body 120 having an outer diameter substantiallyequal to that of the first ring portion 112, and these first ringportions serve as the stimulation electrodes 114 in FIG. 14.

Because second ring portions 116 have a smaller outer diameter, they arecovered by lead body 120; i.e. the surface of the second ring portions116 is covered by a layer of the dielectric lead body material. Thus,the second ring portions are shown in dotted line format in FIG. 19.Alternatively, the electrode may be manufactured with a dielectric layerover the outer surface of the second ring portion. Preferably, the outerdiameter of the dielectric layer would have an outer diametersubstantially equal to the outer diameter of lead body 120.

During an MRI scan, energy is created in the stimulation lead, and someof that energy exits the lead through the stimulation electrodes in theform of current. This current in the stimulation electrodes can causethe temperature of the electrodes to increase to an undesirable level.By increasing the surface area of the stimulation electrodes, thecurrent density in the stimulation electrodes is decreased therebydecreasing the temperature of the electrodes. By utilizing the secondring portion 116, the stimulation electrode has been effectivelyextended to be longer than just the stimulation portion of theelectrode. The extended portion has a smaller diameter and is covered bya thin layer of dielectric material as above described. When lead 104 isimplanted in a patient's body, the second ring portion 116 and thedielectric material thereon form a capacitor with body tissue and/orfluids. The resulting capacitor becomes conductive at MRI frequencies.Thus, at MRI frequencies, the stimulation electrodes have an effectivesurface area larger than the surface area of their respective first ringportions 112 and the resulting lower density current may be safelydissipated into body tissue or fluid.

The material used to cover the second ring portion 116 may be any typeof a biocompatible, non-conductive polymer; preferably a silicone orTeflon type of polymer as these are widely used as lead jackets. Thethickness of the layer (i.e. the difference between the outer radii ofthe first and second ring portions) may be adjusted to select thefrequency at which the second ring portion conducts.

It is an important design consideration that the lead's handlingcharacteristics are not compromised. For example, a lead equipped withthe longer stimulation electrodes shown in FIGS. 16-18 should not beless flexible than previously used leads. To this end, the second ringportion 116 can be made in the form of a bellows 122 shown in FIG. 20.In this manner, second ring portion 116 would be flexible and thereforenot significantly increase the stillness of the distal end of the lead.

In another embodiment, heat generated at MRI frequencies can bedissipated by means of a shunt and a floating electrode. Such asarrangement is shown generally in FIG. 21 which is similar to the leadshown in FIG. 14 where like elements are denoted by line referencenumerals. That is, lead 104 has a plurality of proximal contacts 112 anda plurality of stimulation electrodes 114. However, the lead shown inFIG. 21 also comprises a floating electrode 124 that is placed somewherealong the length of lead 104, preferably proximate the stimulationelectrodes 114. Floating electrode 124 is not used for stimulation butis configured to be capacitively coupled to the stimulation electrodes114 so as to increase the effective surface area of the electrodes inthe high frequency environment of an MRI scan. It should be noted that,if desired, floating electrode 124 may have a layer of dielectricmaterial disposed thereon and thereby be capacitively coupled to thepatient's body.

Capacitive coupling between floating electrode 124 and stimulationelectrodes 114 may be accomplished by configuring a capacitor withineach stimulation electrode 114 (i.e. an electrode capacitor) and thenelectrically coupling the electrode capacitor to floating electrode 124.This may be accomplished using a rolled capacitor construction 126 shownin cross-section in FIG. 22 wherein the outer capacitor roll or plate128 comprises the stimulation electrode and an inner roll or plate 130comprises in inner capacitor plate. The space between the outer roll 128and inner roll 130 may comprise a dielectric material 129 to completethe capacitor. It is then only necessary to electrically couple each ofthe electrode capacitors to floating electrode 124. This may beaccomplished by means of a wire 132 coupled to each electrode capacitor(i.e. to one of the inner or outer plates) and to the floating electrode124 as is shown in FIG. 23. If desired, inner roll or plate 132 maycomprise multiple coils or turns 131 as shown in FIG. 24. The spacebetween coils and plates comprises dielectric material 129.

Alternatively, a ceramic capacitor (e.g. a discoital capacitor) could beused to create the capacitance between floating electrode 124 and astimulation electrode 114. Such an arrangement is shown in FIG. 25. Ascan be seen, a capacitor is comprised of inner and outer plates 134 and136 separated by a ceramic material 138. This capacitor is thenpositioned within stimulation electrode 114 such that plate 134 iselectrically coupled to electrode 114. Each of the electrode/capacitorassemblies thus formed may be electrically coupled together and tofloating electrode 124 by, for example, a single wire 132 as shown inFIG. 23. Discoital capacitors are well known and further discussion isnot deemed necessary; however, the interested reader is referred to U.S.Pat. No. 6,660,116 issued Dec. 9, 2003 and entitled “Capacitive FilteredFeedthrough Array for an Implantable Medical Device”.

FIG. 26 illustrates yet another embodiment of the present invention.Floating contact 124 is provided with a conductive extension 140 thatextends through stimulation electrodes 114 in capacitor formingrelationship therewith. The area between each stimulation electrode 114and extension 140 is occupied by a dielectric material. Thus, at highfrequency (MRI frequencies), the capacitors formed by stimulationelectrodes 114 and extension 140 will conduct induced current tofloating electrode 124 via extension 140. This current is thendissipated into a patient's body tissue or fluid at a low currentdensity, thus reducing unwanted heating of the stimulation electrodes114.

While at least one exemplary embodiment has been presented in theforegoing detailed description of the invention, it should beappreciated that a vast number of variations exist. For example, whilethe invention has been described in connection with medical systems, theinvention is equally applicable to other systems that may be adverselyimpacted in high frequency environments such as is encountered during anMRI scan. It should also be appreciated that the exemplary embodiment orexemplary embodiments are only examples, and are not intended to limitthe scope, applicability, or configuration of the invention in any way.

The foregoing detailed description will provide those skilled in the artwith a convenient road map for implementing an exemplary embodiment ofthe invention, it being understood that various changes may be made inthe function and arrangement of elements described in an exemplaryembodiment without departing from the scope of the invention as setforth in the appended claims.

What is claimed is:
 1. A pulse stimulation system for implantation in apatient's body, the system comprising: a pulse generator; a conductivefilar having a proximal end electrically coupled to said pulse generatorand having a distal end; a lead body for housing said conductive filar;a first electrode coupled to the lead body, electrically coupled to thefilar, and configured to contact the patient's body; and a secondelectrode isolated from the first electrode at a low frequency range andcapacitively coupled to said first electrode at a high frequency rangeand configured to be capacitively coupled to the patient's body at leastat the high frequency range, wherein the second electrode iscapacitively coupled to said first electrode separately from the secondelectrode being capacitively coupled to the patient's body.
 2. Thesystem of claim 1 wherein said second electrode is configured to beisolated from the patient's body at the low frequency range.
 3. Thesystem of claim 2 wherein said first electrode comprises a ring contacthaving a first outer diameter and said second electrode comprises a ringhaving a second outer diameter smaller than said first outer diameter.4. A lead assembly, comprising: a conductive filer having a proximal endelectrically and having a distal end; a lead body for housing saidconductive filer; and a stimulation electrode configured on said leadbody and electrically coupled to the distal end of said filer; afloating electrode that is configured on said lead body and that isisolated from the stimulation electrode at a low frequency range, thefloating electrode comprising: a first section configured tocapacitively couple to the patient's body; and a second sectionelectrically coupled to said first section and configured to becapacitively coupled to the stimulation electrode at a high frequencyrange, wherein the second section is capacitively coupled to thestimulation electrode separately from the first section beingcapacitively coupled to the patient's body.
 5. The lead assembly ofclaim 4 wherein said second section is an extension from said firstsection.
 6. The lead assembly of claim 5 wherein said first sectioncomprises a ring contact having a first outer diameter and said secondsection comprises a ring having a second outer diameter smaller thansaid first outer diameter.
 7. The lead assembly of claim 6 wherein saidsecond section is covered with an insulating material.
 8. The leadassembly of claim 6 wherein said lead body is non-conductive andoverlies said second section.
 9. A pulse stimulation system configuredto be implanted in a patient's body, the system comprising: a pulsegenerator; a conductive filar having a proximal end electrically coupledto said pulse generator and having a distal end; a lead body for housingsaid conductive filer; a first electrode coupled to the lead body,electrically coupled to the filar, and configured to contact thepatient's body; and a second electrode isolated from the first electrodeat a low frequency range and capacitively coupled to said firstelectrode at a high frequency range and configured to be capacitivelycoupled to the patient's body at least at the high frequency range,wherein said second electrode is configured to be isolated from thepatient's body at the low frequency range and wherein said firstelectrode comprises a ring contact having a first outer diameter andsaid second electrode comprises a ring having a second outer diametersmaller than said first outer diameter.
 10. A lead assembly, comprising:a conductive filer having a proximal end electrically and having adistal end; a lead body for housing said conductive filer; a stimulationelectrode configured on said lead body and electrically coupled to thedistal end of said filer; a floating electrode that is configured onsaid lead body and that is isolated from the stimulation electrode at alow frequency range, the floating electrode comprising: a first sectionconfigured to capacitively couple to the patient's body; and a secondsection electrically coupled to said first section and configured to becapacitively coupled to the stimulation electrode at a high frequencyrange, wherein said second section is an extension from said firstsection and wherein said first section comprises a ring contact having afirst outer diameter and said second section comprises a ring having asecond outer diameter smaller than said first outer diameter.